1. Field of the Invention
The present invention relates to a non-cooled infrared image sensor requiring no cooling device, in particular, an infrared image sensor adopting a method of reading a signal by the passage of a bias current.
2. Description of the Related Art
Recently, as thermal infrared image sensors which need no cooling devices, infrared image sensors of vanadium oxide bolometer type and BST (Barium-Strontium-Titanate) pyroelectric type have been commercialized. These products comprise heat-sensitive parts which absorb infrared radiation and raise the temperature thereof, support legs for thermally separating the heat-sensitive parts from a silicon substrate, and horizontal address lines and vertical signal lines and the like for selecting pixels formed of the heat-sensitive parts. Each heat-sensitive part absorbs infrared radiation from a subject, and the rise in the temperature of the heat-sensitive part in absorption is sensed on the basis of change in the resistance or capacitance. Therefore, the support legs supporting the heat-sensitive part are configured to have a small cross section and a long length, to enhance their heat-insulating effects.
Among the products, in the method called “bolometer type” in which the rise in the temperature of the heat-sensitive part is detected on the basis of change in resistance, a bias current is fed for a certain period of time, to accumulate (integrate) signals by infrared radiation. Therefore, during the signal accumulating period, the heat-sensitive parts are heated by Joule heat generated by the bias current. The rise in the temperature caused by the Joule heat is greater than the rise in the temperature caused by absorption of infrared radiation from the subject, thus most of signals from the heat-sensitive parts is occupied by noise generated by the Joule heat. To cancel the noise, there have been proposed a method, in which an integrator circuit is provided for each column and charge for noise generated by Joule heat is removed through a reference resistor, and the like (for example, please refer to Jpn. Pat. Appln. KOKAI Pub. No. 9-284651).
However, such methods have the following problem. Specifically, the above canceling mechanism does not work, in the case where the temperature of the package on which the chip is mounted has changed, in particular, in the case where the temperature of the chip rapidly changes, such as directly after start of driving the chip. Further, much heat is radiated from the whole heat-sensitive parts, and contributes to change in the temperature of the chip.
As described above, in conventional thermal infrared image sensors, it is difficult to remove noise components generated by Joule heat in the case where the temperature of the chip rapidly changes, and this is a main cause of lowering the reliability of the sensors. Therefore, it has been desired to realize an infrared image sensor which is inexpensive and highly reliable and can effectively remove noise components accompanied by change in the temperature of the chip.